异常java.lang.ClassCastException

Question

我试图从表中只读取两列（一个是整数，另一个是字符串）并获取结果列表。我正在使用EntityManger读取表，如下面的代码所示。此代码正确执行，没有异常出现。

@SuppressWarnings("unchecked")
public List<BusinessProcessIdAndName> getBusinessProcessList(int level) {
    List<BusinessProcessIdAndName> businessProcessTableList = new        ArrayList<BusinessProcessIdAndName>();
    EntityManager em = null;
    try {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory(ApplicationConstants.DERBY_PERSISTENCE_UNIT_NAME);
        em = emf.createEntityManager();
        EntityTransaction et = em.getTransaction();
        et.begin();
        Query query = em.createQuery("select t.businessProcessId, t.businessProcessName from BusinessProcessTable t");
        businessProcessTableList = query.getResultList();
        // The line below displays the correct result, The logger is basically a System.out.println  
        logger.debug(businessProcessTableList.size());
    } 
    catch(Exception e) {
        logger.debug("Exception Caught while getting BP List: " + e);
    }
    return businessProcessTableList;
}

BusinessProcessIdAndName类如下所示

public class BusinessProcessIdAndName {

    private Integer businessProcessId;
    private String businessProcessName;

    public Integer getBusinessProcessId() {
        return businessProcessId;
    }

    public void setBusinessProcessId(Integer businessProcessId) {
        this.businessProcessId = businessProcessId;
    }

    public String getBusinessProcessName() {
        return businessProcessName;
    }

    public void setBusinessProcessName(String businessProcessName) {
        this.businessProcessName = businessProcessName;
    }

}

然后在托管Bean中，我使用上面代码的结果如下所示。在这里我得到了异常

JAVAlang.ClassCastException:[Ljava.lang.Object；与com.ewt.ewtalmutil.Object.BusinessProcessIdAndName不兼容

我知道这个例外说明对象不匹配，它们不兼容，但我认为我的对象应该兼容，请告诉我哪里错了，正确的方法是什么。

public SelectCriteriaBean () {
    logger.entering(CLASS_NAME);
    this.businessProcessLevelZeroList = new ArrayList<SelectItem>();
    List<BusinessProcessIdAndName> tempBusinessProcessLevelZeroList = new BusinessProcessTableManager().getBusinessProcessList(0);
    // The line below also displays correct result
    logger.debug(tempBusinessProcessLevelZeroList.size());
    // The line below gives Exception: java.lang.ClassCastException: [Ljava.lang.Object; incompatible with com.ewt.ewtalmutil.object.BusinessProcessIdAndName
    try {
        Iterator<BusinessProcessIdAndName> iterator = tempBusinessProcessLevelZeroList.iterator();
    }
    catch (Exception e) {
        logger.debug("Exception: " + e);
    }
    while (iterator.hasNext()) {
        BusinessProcessIdAndName businessProcessIdAndName = new BusinessProcessIdAndName();
        businessProcessIdAndName = iterator.next();
        // The Exception/Error comes in the line below 
        String businessProcessName = businessProcessIdAndName.getBusinessProcessName();
        int businessProcessId = businessProcessIdAndName.getBusinessProcessId();
        String businessProcessIdString = String.valueOf(businessProcessId);
        SelectItem item = new SelectItem(businessProcessIdString,businessProcessName);
        businessProcessLevelZeroList.add(item);
    }
    setBusinessProcessLevelOneListRendered(false);
    setAddBusinessProcessRendered(false);
    logger.exiting(CLASS_NAME);
}

Answer 1

考虑以下三个例子。

• 如果试图选择表中的所有列。然后你必须像下面的代码那样编写你的程序

如

Query q = em.createQuery("SELECT t FROM BusinessProcessTable t");    
List<BusinessProcessTable > result = q.getResultList();

• 如果您尝试获取一个表的所有列。然后您必须像下面的代码一样编写您的程序。

如

Query q1 = em.createQuery("SELECT t FROM BusinessProcessTable t WHERE t.id = :id");
q1.setParameter("id", "4711");
BusinessProcessTable e = (BusinessProcessTable )q1.getSingleResult();

• 如果您试图获取表中选择列的所有记录，那么返回类型将是对象列表。你应该像下面的代码那样编写你的程序

如

 Query q1 = em.createQuery("SELECT t.name, t.salary FROM BusinessProcessTable t");
        List<Object[]> result1 = q1.getResultList();
        for (Object[] resultElement : result1) {
            String name = (String)resultElement[0];
            Double salary = (Double)resultElement[1];
            ...
        }